16730

The circumgalactic medium (CGM) mediates a galaxy.s accretion and feedback and may hold most of the metals it has ever produced. These findings derive from aggregating over ensembles of halos observed one galaxy at a time comma and so they are limited in their ability to capture the rich density comma temperature comma and kinematic structure that simulations predict. Because its halo spans 30 degrees on the sky comma Andromeda (M31) provides a unique opportunity to map the CGM of a single L* galaxy with dozens of sightlines and to relate the gas mass comma metallicity comma and kinematics to its detailed stellar populations resolved by complementary surveys. We propose to map the inner M31 CGM within 0.25 Rvir using 11 new QSOs and 2 M31 O stars comma a region too sparsely sampled to characterize the most complex and influential region for galaxy evolution. When combined with existing M31 samples at larger radii comma this unique program will enable compelling science goalsdoublePoint 1) to map ongoing accretion and feedback at the disk-halo interface comma 2) to fully characterize the CGM from the disk to 2 Rvir comma 3) to complete the CGM metals census that constrains outflows from the disk comma and 4) to test feedback predictions from the latest zoom-in simulations of galaxy formation. Our program will also trace M31.s assembly history with deep parallel images of its stellar halo. This program will yield valuable new maps of the disk-halo connection comma where CGM becomes ISM comma in the only galaxy for which the effects of gas flows - such as chemical enrichment by specific feedback events - can be measured at this level of detail. Our study of M31 will be the most complete picture of a galaxy.s baryon cycle for the foreseeable future.